Thioethers (organic sulfides) having the structure R'--S--R where R,R' may be alkyl, aryl, or arylalkyl and where any alkyl chain may be interrupted by an oxygen atom have long been known to form complexes with the transition group metals, especially with the metals of groups 8, 1b, and 2b. Several complexes of the form MX.sub.2 (RSR').sub.2 wherein X is a halide group have been described in the literature. Further, the affinity of the various metals for the R'--S--R group varies. For example, Ag, Zn, and divalent Cu ions form stronger halide complexes than do thioether complexes in aqueous media while monovalent Cu and divalent Hg have roughly equal ligand strengths for both the chloride and thioether complexes in aqueous media. In comparison to the above, the platinum group metals and gold form substantially stronger thioether complexes than do the corresponding chloro complexes.
Thus one could expect that thioethers could be employed to selectively complex the platinum group metals and gold in chloride media in the presence of other transition elements and that a solvent extractant or ion-exchange resin could be based on a thioether functional group to extract the platinum group metals and gold as a group. R. I. Edwards reports in U.S. Pat. No. 3,985,552 that such in fact is the case and ion-exchange resins with the active group --CH.sub.2 --S--CH.sub.3 were employed and found to extract Pt, Pd, Au, Rh, and Ir from hydrochloric acid media. He also reports that di-n-octyl sulfide was found to extract only Pd and Au from hydrochloric acid media for this group of elements. Pt, Rh, Ru, and Ir was not extracted at all employing the di-n-octyl sulfide. He further found that while gold was extracted extremely quickly into the organic phase, palladium was extracted relatively slowly. This gave rise to the selectivity of solvent extraction of palladium and gold as well as gold from palladium utilizing the di-n-octyl sulfide or other thioether solvent extractants as reported in the aforementioned patent having the formula R'--S--R.
While the invention as set forth in U.S. Pat. No. 3,985,552, which patent is incorporated herein by reference, is useful for the separation of palladium and gold from base metals (i.e., any metal impurities other than palladium group metals or gold) as well as from other platinum group metals, practical use of the separation technique employing commercial grade thioethers has been found to be limited. In particular, if one wants to obtain a high purity (.gtoreq.99.95%) palladium and/or gold the method has not been suitable. The reason for this limitation is that, a problem exists during the stripping operation after solvent extraction in that while following the method as set forth in the patent, an emulsion which takes several hours to disassociate tends to form upon extraction. This emulsion in and of itself limits the practical use of the technique due to the time required for the emulsion to break up so as to allow phase separation. However, even when the macro-emulsion appears to be broken and two phases formed, a micro-emulsion remains dispersed within the two phases which carries impurity ions along with it causing contamination and loss of solvent.
We have now discovered the cause of the emulsion formation and an improved method of extracting palladium and/or gold from base metals employing the thioether extractants which includes a step of treating the thioether prior to extraction of the palladium and/or gold so as to prevent the formation of a stable emulsion by removing the emulsion causing constituent from the thioether.